Broad band spiral antenna

ABSTRACT

Antenna elements for substantially circularly-polarized electromagnetic energy are shown. The disclosed elements, formed on a tapered form having an elliptical cross section, are particularly well suited for use as elements in an antenna array because such a cross-sectional shape permits individual elements to be more closely positioned with respect to each other, thereby raising the frequency of the electromagnetic energy at which grating lobes occur. In a second embodiment the eccentricity of the tapered form is relatively large, so the greater part of each antenna turn lies in a plane. Such a configuration radiates substantially linearly polarized radio frequency energy.

United States Patent Monser et al.

[4 1 Feb. 26, 1974 [54] BROAD BAND SPIRAL ANTENNA [75] Inventors: GeorgeJ. Monser; John R.

Ehrhardt, both of Santa Barbara,

Calif.

[73] Assignee: Raytheon Company, Lexington,

Mass.

[22] Filed: Oct. 12, 1972 [2]] Appl. No.: 297,112

Related U.S. Application Data [63] Continuation-impart of Ser. No.142,223, May 11,

[52] US. Cl. 343/895, 343/854 [51] Int. Cl. H01q l/36 [58] Field ofSearch 343/895, 908, 854

[56] References Cited UNITED STATES PATENTS 3,587,106 6/1971 Crooks etal. 343/895 3,454,951 7/1969 Patterson et al 343/895 3,019,439 l/l962Reis et al..... 343/895 3,508,269 4/1970 Snyder 343/895 2,919,44212/1959 Nussbaum 343/895 AMPLIFIER AMPLIFIER PARALLEL PLATE LENS FOREIGNPATENTS OR APPLlCATlONS 411,888 7/1945 ltaly 343/895 PrimaryExaminer-E1i Lieberman Attorney, Agent, or Firm-Philip J. McFarland;Joseph D. Pannone ABSTRACT Antenna elements for substantiallycircularlypolarized electromagnetic energy are shown. The disclosedelements, formed on a tapered form having an elliptical cross section,are particularly well suited for use as elements in an antenna arraybecause such a cross-sectional shape permits individual elements to bemore closely positioned with respect to each other, thereby raising thefrequency of the electromagnetic energy at which grating lobes occur.

In a second embodiment the eccentricity of the tapered form isrelatively large, so the greater part of each antenna turn lies in aplane. Such a configuration radiates substantially linearly polarizedradio frequency energy.

5 Claims, 4 Drawing Figures ANTENNA PATENT'ED FEB26 I974 SHEU .1 [If 2AMPLIFIER AMPLIFIER F/GJ ANTENNA nvvewrms GEORGE .1. a/ JOHN R. EHRHARDTPATENTED FEBEB I974 SHEET 2 [IF 2 BROAD BANDSPIRAL ANTENNA Thisapplication is a Continuation-ln-Part of the copending applicationentitled Broad Band Antenna, Ser. No. 142,223, now abandoned, filed MayI l, 1971 by George J. Monser and John R. Ehrhardt and assigned to thesame assignee as this application.

BACKGROUND OF THE INVENTION This invention pertains generally to antennaarrays for radio frequency energy and particularly to antenna arrays forcircularly or linearly polarized radio frequency energy.

It is known in the art that multi-beam antenna arrays may be arranged toproduce simultaneously existing beams of electromagnetic energy havingcircular polarization. Thus, it is known that a circular polarizerassembly may be disposed in the path of the electromagnetic energypassing to or from an array having antenna elements which form eitherplane-polarized electromagnetic energy for transmission or are sensitiveto plane-polarized energy on reception. It is also known thatcircularly-polarized electromagnetic energy may be transmitted from, orreceived by, a multi-beam array without a polarizer assembly if antennaelements are conventional helices.

Unfortunately, known circular or linear polarizer assemblies limit thebandwidth and scan capabilities of multibeam antennas. That. is, becausesuch polarizers must be designed and fabricated to operate onelectromagnetic energy of a particular frequency or several discretefrequencies and because they must, of necessity, absorb some of theradiated energy, their effectiveness decreases with changes in frequencyfrom the design frequency. With increased scan angles the reflections ofthe external polarizers greatly reduce the effectiveness, particularlywith regard to power transmission.

The use of conventional bifilar helical antenna elements avoids, to alarge extent, the power limitations imposed by wide scan angles and, toa lesser extent, the limitations imposed by frequency changes onmultibeam array antennas sensitive to circularly-polarized energy.However, in a multi-beam antenna array which is desired to be broad-band(meaning an antenna of such type which is useful over, say, an octave offrequency change) the use of conventional cylindrical bifilar helicalantenna elements presents difficulty. Thus, it has been found in suchapplications that grating lobes appear (at the higher end of the band)in the antenna pattern of an array using conventional bifilar helicalantenna elements. Such lobes, of course, may not be tolerated inoperational equipments because of their adverse effect on angularaccuracy and where maximum directive gain is required.

SUMMARY OF THE INVENTION AND DESCRIPTION OF THE DRAWINGS Therefore, itis the main object of this invention to provide an improved helicalantenna element for use primarily in a multi-beam antenna array, suchimproved element being adapted substantially to circularly polarizedelectromagnetic energy and, at the same time, avoid the formation ofgrating lobes in the antenna pattern of such array, under wide scanoperation on the order of 1 60 from the array normal.

Another object of this invention is to provide improved helical antennaelements which, when incorporated in a multi-beam antenna array,increase the bandwidth of such array.

Another object of this invention is to provide improved helical antennaelements which, when incorporated in a multi-beam antenna array, causelinearly polarized energy to be radiated.

These and other objects of this invention are attained generally byproviding a tapered helical antenna element which is supported on a formhaving a cross section conical near the apex and tapering tosubstantially an elliptical shape. In a second embodiment, the form is,in cross-section, an ellipse with a large eccentricity. For a morecomplete understanding of this invention, reference is now made to thefollowing description of a preferred embodiment as illustrated in theaccompanying drawing in which:

FIG. 1 is a representation, partially in block diagram form andpartially in the form of a sketch of the contemplated antenna elementused as an array element in a multi-beam antenna array of a transponderusing this invention;

FIG. 2 is an isometric view of a single one of the antenna elementsshown in FIG. 1; and

FIG. 3 is an isometric view of an alternative embodiment of thisinvention to radiate substantially linearly polarized radio frequencyenergy;

F IG. 4 is an isometric view of an alternative embodiment of thisinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Before referring to thedrawings, it will be advantageous to note that our inventive conceptsare applicable either to phased antenna arrays (meaning arrays having aseparate adjustable phase shifter in the path of electromagnetic energybetween a single focal point and each one of a matrix of antennaelements, the adjustment of each such phase shifter determining thedirection of a single beam relative to the matrix of antenna elements)or to multi-beam antenna arrays (meaning arrays having anelectromagnetic lens arrangement associated with a matrix of antennaelements, such lens arrangement determining the direction of each one ofa plurality of simultaneously existing beams relative to the matrix ofantenna elements). For expository purposes, however, we have chosen toshow a preferred embodiment of our invention in a multi-beam antennaarray.

With the foregoing in mind it may be seen in FIG. 1 that a transponderaccording to our invention includes a multi-beam receiving antenna 11and a similar multibeam transmitting antenna 13 (the former beingdescribed in detail hereinafter), individual ones of the antennaelements 15a through 15n being connected through transmission lines 17athrough 17n to a corresponding coupling point 19a through 19n of aparallelplate lens 2lr. It is known, for example as shown by W. Rotmanand RF. Turner in their paper entitled Wide Angle Microwave Lens forLine Source Applications in the Transactions of the Institute ofElectrical and Electronic Engineers, Nov., 1963, pp. 623-632 publishedby the Institute of Electrical and Electronic Engineers, Inc., New York,N.Y., that an array of antenna elements connected through transmissionlines of chosen lengths to a parallel-plate lens may be arranged tofocus a planar wave of electromagnetic energy at a point along an arc ofbest'focus, the particular point on such are at which such energy isfocused being determined by the direction of the origin of the planarwavefront. Thus, by connecting each one of a group of amplifiers 23athrough 23x to a different point 25a through 25):, an output signal fromany one of such amplifiers is indicative of a radio frequency signalfrom a particular direction. it is noted here that the number ofdifferent points 25a through 25x (and, therefore, the number ofamplifiers 23a through 23x) may be varied as desired to change thenumber of beams. in any event, the output terminal of each one of theamplifiers 23a through 23x is connected to a corresponding point 27athrough 27x on the arc of best focus of a parallelplate lens 291. Thelatter is similar to the parallel plate lens 21r. in like manner,transmission lines 31a through 3ln, each of which has the same length asa corresponding one of the transmission lines 17a through 1711, isconnected between a point 33a through 33m to an antenna element 35athrough 35n of the multi-beam transmitting antenna array 13.

Referring now to FIGS. 1 and 2 it may be seen that each one of theantenna elements 35a through 35n is a modified conical-helical antennawith a single winding, the base 37 of each such winding being mounted ona metallic ground plate (not numbered) in any convenient manner. As isknown, a single conical-helical antenna mounted on a metallic groundplate has a relatively broad antenna pattern, say in the order of 70 to80 between half-power points, with nulls orthogonal to the axis of thehelix. it is also known that, as a transmitting antenna, a singleconical-helical antenna radiates circularly-polarized radio frequencyenergy and that, as a receiving antenna, an antenna of suchconfiguration is responsive to circularly polarized radio frequencyenergy. The sense of polarization, i.e. whether left-hand or right-hand,to which a conical-helical antenna element is responsive depends on thedirection of the active winding of the element. It is also known that asingle conical-helical antenna element may be made to be operative overa relatively wide band of frequencies, meaning more than an octave.Thus, it is known that a single conical-helical antenna elementsubstantially maintains its efficiency from a lower frequency limitdetermined by the diameter of its base to an upper frequency limitdetermined by the pitch of its active winding. When, however,conical-helical antenna elements are disposed together in an array, theyare subject to the same strictures as apply to any antenna elements soused. That is, in order to avoid the formation of grating lobes, it isnecessary that antenna elements in an array be disposed so that thedistance between phase centers of adjacent elements be less than onehalfwavelength of the radio frequency energy passing to or from suchelements. Such a limitation obviously determines the upper end of theband of frequencies within which any array of antenna elements may beused.

With the foregoing in mind it may be seen that the antenna element wecontemplate, as illustrated and clearly shown in FIG. 2, comprises abase 37 having a generally elliptical shape on which notched spacers39a, 39b, 39c, 39d are mounted as shown to form a generally conicalwinding support (not numbered) for a wire winding 41. The pitch of thewire winding 41 preferably varies as shown. It is noted here in passingthat although the pitch of the wire winding 41 may vary, the locus ofsuch winding on the generally conical winding support is substantiallyhelical. The feed for the wire winding 41 is effected, through aconventional matching circuit 43 and connector 45 mounted in the centerof the element, from a transmission line, said transmission line 17a(FIG. 1).

When a number of the antenna elements through 15n are disposed along ametallic ground plate, as in FIG. 1, to form a linear array, suchelements are arranged so that the major axes of successive ones thereofare parallel to each other as shown. Thus, the phase center ofsuccessive antenna elements 15a through 15m are, in the plane of theelements, as close together as possible. The exact spacing betweensuccessive antenna elements 15a through 15n is, of course, dependentupon the length of the minor axes of such elements. In any event, itwill be clear that the spacing between successive antenna elements 15athrough l5n will be smaller than would be possible with conventionalconical-helical antenna elements having diameters equal to the majoraxes of the disclosed antenna element. Consequently, the upper limit ofthe frequency band in which an array according to our inventive conceptswill be higher than a conventional conical-helical antenna array havingthe same lower limit. It will be recognized that the polarization of theradio frequency energy to which our antenna array is best fitted iselliptical rather than circular, the ellipticity of such polarizationcorresponding to the eccentricity of the base 37 of each antenna element15a through l5n. We have found, however, that the extension of the upperlimit of the frequency band to which our array is responsive more thancompensates for any degradation of polarity in many practical systems.

Referring now to FIG. 3, it may be seen that the eccentricity of thebase 37 (FIG. 2) may be increased so as to make the cross-section of thewire windings appear almost rectangular. The polarization of the energyradiated by such a modified antenna element is, then, substantiallylinear. Thus, the base 37' of FIG. 3 is an extremely eccentric ellipse,causing a tapered support 50 to be similarly eccentric. Consequently,the greater part of each one of the helical turns of the bifilar wirewindings 41, 41' about the tapered support 50 is substantially flat.Wire winding 41 here is an extension of the center conductor of acoaxial line 52 helically wound around the tapered support 50 as shownand wire winding 41 here is a wire connected in any convenient manner tothe shield of the coaxial line 52 and interlaced with wire winding 41.Alternatively, wire windings 41, 41 could be wires coupled to the armsof a hybrid junction, such junction being fed by a transmission line.

The dimensions of the base 37 and of the tapered support 50 and thenumber of helical turns of the wire winding 41 may be varied within widelimits. it is necessary only that the length of each turn of the wirewinding 41 be equal to or greater than the wavelength of the radiofrequency energy to be radiated.

Having described two embodiments of our invention, it will be apparentto one of skill in the art that many changes and modifications may bemade thereto without departing from our inventive concepts. For example,each antenna element could incorporate a conventional bifilar winding inplace of the single winding as shown in FIG. 2. in addition, the shapeof the base could be changed from an ellipse to a hexagonal shape, asshown in FIG. 4, if a planar array is desired; such change would makethe distance between the phase centers of adjacent antenna elements assmall as possible in'orthogonal planes to prevent grating lobes fromforming in either the azimuthal or elevation planes. It is felt,therefore, that this invention should not be restricted to its disclosedembodiments but rather should be limited only by the spirit and scope ofthe appended claims.

What is claimed is: 1. An antenna array for substantially circularlypolarized radio frequency energy, such array comprising:

a. a plurality of like antenna elements, each one thereof including aphase center and a wire radiators, the active portion thereof beinghelically wound around the frustum of a conic-like supporting memberhaving an elliptical cross section; and

b. means for mounting each one of the plurality of like antenna elementsin juxtaposition one to another to form an antenna array wherein themajor axes of the elliptical cross sections of the conic-like supportingmembers are parallel one to another, the phase centers of adjacentelements being separated by less than one-half the wavelength of theradio frequency energy.

2. An antenna array as in claim 1 wherein the wire radiator comprisesbifilar windings.

3. An antenna array as in claim 2 wherein the end of each one of thebifilar windings adjacent to the top of the frustum is passed throughthe center thereof.

4. A planar antenna array for substantially circularlypolarized radiofrequency energy, such array comprismg:

a. a plurality of like antenna elements, each one thereof having a phasecenter and being helically wound around the frustum of a pyramid havinga hexagonal base; and

b. means for mounting each one of the plurality of like antenna elementsto abut one to another to form an antenna array, the phase centers ofadjacent elements being separated by less than one-half the wavelengthof the radio frequency energy.

5. An antenna array for radio frequency energy comprising:

a. a plurality of like antenna elements, each one thereof including aphase center and a wire radiator having its active portion helicallywound around a tapered supporting member, any cross section of suchmember being a highly eccentric ellipse to cause radio frequency energypropagated from the active portion of the wire radiator to besubstantially plane polarized; and

b. means for mounting each one of the plurality of like antenna elementsin juxtaposition one to another to form the antenna array, the phasecenters of adjacent elements being separated by less than one-half thewavelength of the radio frequency energy.

1. An antenna array for substantially circularly polarized radiofrequency energy, such array comprising: a. a plurality of like antennaelements, each one thereof including a phase center and a wireradiators, the active portion thereof being helically wound around thefrustum of a conic-like supporting member hAving an elliptical crosssection; and b. means for mounting each one of the plurality of likeantenna elements in juxtaposition one to another to form an antennaarray wherein the major axes of the elliptical cross sections of theconic-like supporting members are parallel one to another, the phasecenters of adjacent elements being separated by less than one-half thewavelength of the radio frequency energy.
 2. An antenna array as inclaim 1 wherein the wire radiator comprises bifilar windings.
 3. Anantenna array as in claim 2 wherein the end of each one of the bifilarwindings adjacent to the top of the frustum is passed through the centerthereof.
 4. A planar antenna array for substantiallycircularly-polarized radio frequency energy, such array comprising: a. aplurality of like antenna elements, each one thereof having a phasecenter and being helically wound around the frustum of a pyramid havinga hexagonal base; and b. means for mounting each one of the plurality oflike antenna elements to abut one to another to form an antenna array,the phase centers of adjacent elements being separated by less thanone-half the wavelength of the radio frequency energy.
 5. An antennaarray for radio frequency energy comprising: a. a plurality of likeantenna elements, each one thereof including a phase center and a wireradiator having its active portion helically wound around a taperedsupporting member, any cross section of such member being a highlyeccentric ellipse to cause radio frequency energy propagated from theactive portion of the wire radiator to be substantially plane polarized;and b. means for mounting each one of the plurality of like antennaelements in juxtaposition one to another to form the antenna array, thephase centers of adjacent elements being separated by less than one-halfthe wavelength of the radio frequency energy.